Adult Schistosoma mansoni were removed from hamsters and exposed in vitro to H3-thymidine. Worms were processed for autoradiography to determine what cells became labeled. Good labeling was achieved in 2- hour exposures and was found mainly over nuclei of gonial cells in the ovary, testes, and vitelline glands. Only a few nuclei of scattered parenchymal cells showed labeling. Worms were labeled in vitro and transplanted to the portal vein of uninfected hamsters for period of up to 10 days. Autoradiograms of these worms show that by 8 days sperm are labeled and were transferred to female worms. Thus insemination is a frequent process. Labeled oogonia had not moved appreciably through the ovary even after 10 days in vivo. The thymidine label of vitelline cells in 10 day transplanted worms was much reduced over that found in the initial exposure, indicating dilution of label through nuclear division. Similar timing studies are underway with adult Schistosoma japonicum. Adults of S. mansoni maintained intraperitoneally were dead after 3 days and showed signs of a cellular reaction on the tegument. Worms maintained in vitro are now being compared for timing of reproductive processes to those maintained in vivo.